But his potential move to invoke the “nuclear option” is raising a bigger and more sweeping question that could have huge consequences for future presidents of both parties: Is this the beginning of the end of the filibuster? If the filibuster goes, the Senate would lose a crucial check on majority rights — and it could start looking very much like the House, where the majority always gets its way.

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For years, the filibuster has been increasingly used as a tool to block, delay and frustrate the will of the majority party to push through its agenda. While the filibuster has been changed periodically over the years, senators have never successfully made good on their threat to impose the “nuclear option” — changing Senate cloture rules by 51 votes, rather than 67 — for fear it would hurt them one day when they were back in the minority.

On Monday, Reid informed President Barack Obama about his intention to use the nuclear option if no deal is struck, sources said, and Obama signaled he would support the effort.

Senate Minority Leader Mitch McConnell (R-Ky.), who is trying to head off the high-stakes fight, privately reached out to Vice President Joe Biden, but it’s expected that Biden would vote with Democrats in case there’s a 50-50 tie.

The crisis could still be averted. Reid signaled that he would drop the threat of the nuclear option if Republicans ended their filibusters on pending Obama nominees .

But senators in both parties agreed Thursday that if Reid moves to change the rules by 51 votes, it would be used by the majority in the future to further weaken the filibuster, potentially eliminating the potent procedural weapon altogether one day. While Democrats said they were willing to roll the dice on the nuclear option, believing the GOP would go that route anyway when they get back in the majority, Republicans said Reid’s move all but assured a continued weakening — and eventual demolition — of the filibuster.

In a closed-door caucus meeting Thursday, Reid began by apologizing to his colleagues for cutting bipartisan deals to avert the nuclear option, including at the beginning of this year. And the Nevada Democrat complained that he allowed votes on scores of conservative nominees under former President George W. Bush after a bipartisan coalition headed off the nuclear option in 2005. But Reid said it had been the right thing to do because Bush had won a second term in the White House.

Now, Reid argued, times have changed.

“I ate sh— on some of those nominees,” Reid told his colleagues, according to sources who were present.

On Thursday, the Senate continued to inch closer to a battle that could have dramatic implications for the institution.

“I don’t know how you open that door and not go to the next level. First, it’s executive nominations, next thing it’ll be judicial nominations, then it will be legislative filibusters,” said South Dakota Sen. John Thune, the No. 3 Senate Republican. “The precedent they set here will be not only long-lasting but far-reaching.”

Under Reid’s proposal, the Senate would no longer need 60 votes to break a filibuster on executive branch positions, bringing down that threshold to 51 votes. Senators would face a 60-vote requirement to end filibusters on legislation and judicial nominees.

At the heart of the fight are a handful of nominees Reid has been pushing to confirm. On Thursday, he set up procedural votes on a slate of nominees, but Republicans are poised to block Richard Cordray to lead the Consumer Financial Protection Bureau, as well as nominees for the National Labor Relations Board. These nominees are at the center of an ongoing legal struggle over the legality of Obama’s use of recess appointment power to place them at the agencies.